1. Field of the Invention
This invention relates to an improved high modulus honeycomb comprising a matrix resin and paper that is made with a high melt point thermoplastic fiber binder that can withstand certain honeycomb manufacturing steps that utilize hot water or steam treatment. The thermoplastic fiber binder has a melting temperature above the curing temperature of the matrix resin, a glass transition temperature of greater than 100° C., and the weight average molecular weight of the thermoplastic polymer in the thermoplastic fibers changes 20% or less after being maintained for 10 minutes at the melting temperature.
2. Description of Related Art
Paper-based honeycomb is typically formed by (1) applying adhesive resin to sheets of paper along predetermined lines, called node lines, (2) adhering several sheets of paper along these node lines to form a stack, with the node lines of each sheet offset to the adjacent sheets, (3) expanding the stack to form a honeycomb having defined cell walls, (4) impregnating the cell walls of the honeycomb with structural resin by submerging the honeycomb in a liquid resin, and (5) curing the resin with heat. U.S. Pat. No. 5,137,768 to Lin, U.S. Pat. No. 5,789,059 to Nomoto, and U.S. Pat. No. 6,544,622 to Nomoto disclose honeycombs made from sheets made from high modulus para-aramid materials. These honeycombs are highly prized for structural applications due to their high stiffness and high strength to weight ratio. Generally these honeycombs are made from papers comprising para-aramid fibers, pulp, and/or other fibrous materials plus a binder. These patents disclose a variety of binders may be used and the use of low cost thermoplastic binders, such as polyester type binders, have been found to create a paper that has a very open or porous structure, allowing rapid impregnation of thermoset structural resin into the honeycomb walls when dipped in resin.
While not specifically used in honeycomb, U.S. Pat. Nos. 6,551,456 and 6,458,244 to Wang et al. and Japanese Patent Application Publication 61-58,193 to Nishimura et al., disclose papers made from aramid fibers combined with polyester fibers. It is believed that these papers also have a very open or porous structure, allowing rapid impregnation of thermoset structural resins.
British Patent No. 803,259 describes a process by which cellular structures (honeycombs) can be made from cellulosic sheets such as kraft paper by subjecting the stack of sheets with a vapor such as steam during or after expanding the sheets into cells. The '259 patent further describes the need to provide some support for honeycombs made with low wet strength paper while such papers dry. It is desired to use steam treatment process to help expand into honeycomb structures adhered paper sheets of high modulus fibrous materials and thermoplastic binders, however, the high temperature of the steam can either soften the thermoplastic binder causing undesired deformation of the honeycomb, or can liquefy the thermoplastic, which can flow and deposit on manufacturing equipment or in a worst case, further adhere the paper sheets together. In either case, the nature of the thermoplastic binder prevents the manufacture of a high quality honeycomb. Therefore, what is needed is a thermoplastic binder that provides adequate paper binding strength while at the same time can reliably withstand steam treatment during honeycomb manufacture.